1. Field of the Invention
The present invention relates to a manufacturing method for a semiconductor device and, in particular, to a manufacturing method for a device having silicon oxide films with at least two different thicknesses, which includes a process for improving a reliability of the silicon oxide film.
2. Description of the Related Art
Up to now, in manufacturing a MOS transistor having gate oxide films with two different thicknesses on a semiconductor substrate, the following processes have been adopted for improving the reliability of one of the gate oxide films.
First, as shown in FIG. 2A, an element isolation film 10 is formed on a silicon semiconductor substrate 9 using any well-known technique. After that, a first gate oxide (insulating) film 11 is formed, for example, through thermal oxidation of the silicon substrate. Active regions are formed in two or more regions owing to the existence of the element isolation film 10.
Next, as shown in FIG. 2B, the first gate insulating film 11 is subjected to nitriding. In general, through the nitriding, nitrogen is segregated at an interface between the silicon substrate and the gate oxide film as a silicon oxynitride 12 to cover an interface state or a trap, resulting in an improved reliability of the first gate insulating film.
Subsequently, as shown in FIG. 2C, a mask member l3, which is used for opening at least a portion where a second gate oxide film is to be formed, is made of, for example, a photoresist film using any well-known technique. Thereafter, a part of the first gate oxide film is selectively removed.
In general, the removal of the first gate oxide film is made using hydrofluoric acid. The application of hydrofluoric acid alone is insufficient for removing the silicon oxynitride (film) 12 formed at the interface between the silicon substrate and the gate oxide film. As a result, the silicon oxynitride 12 remains on the silicon substrate surface.
Next, as shown in FIG. 2D, the mask member 13 is removed, followed by forming a second gate oxide film 14, for example, through the thermal oxidation of the silicon substrate.
Subsequently, as shown in FIG. 2E, a gate electrode film 15 and source/drain 16 of a transistor are formed using any well-known technique. Through the above processes, the MOS transistor is manufactured.
According to a conventional manufacturing method, the second gate oxide film is formed while the silicon oxynitride formed upon nitriding the first gate oxide film remains on the silicon substrate surface. As a result, in the case of forming the second gate oxide film, in particular, through the thermal oxidation, the silicon oxynitride acts thereon as an inhibitor against the oxidation, which causes such a problem as a remarkable deterioration in reliability of the second gate oxide film.